In typical cellular wireless communications, each wireless communication device (WCD) (e.g., cell phone, personal digital assistant, wirelessly equipped personal computer, etc.) subscribes to service from a given cellular wireless service provider known as the device's home system. In practice, a home system will operate one or more radio access networks (RANs) including base transceiver stations (BTSs) that radiate to define wireless coverage areas in which the subscriber WCDs can operate. Through each BTS (and corresponding RAN), a WCD can obtain connectivity to broader networks such as the public switched telephone network (PSTN) and the Internet. Further, each RAN may include one or more radio network controllers (RNCs) or the like, which may be integrated with or otherwise in communication with the BTSs, and which may include or be in communication with a switch or gateway that provides connectivity with one or more transport networks. Conveniently with this arrangement, a WCD that is positioned within coverage of the RAN can then communicate with a BTS and in turn, via the BTS, with other served devices or with other entities on the transport network.
When a subscriber WCD enters into coverage of its home system, the subscriber WCD may register with the home system (through a BTS, for instance) and may then engage in wireless communication service via the home system's network, and the home system may accordingly charge or debit a service account of the device.
Although a typical wireless service provider may strive to offer wireless coverage throughout a region, such as nationally in the United States for instance, the service provider may not actually operate RANs in all locations throughout the region. To fill in gaps where the service provider does not operate its own RAN, and thus where the service provider does not itself provide coverage, the service provider will typically enter into roaming agreements with other service providers (sometimes referred to as “roaming systems”). Under a roaming agreement, another service provider may agree to provide service to the home system's subscriber WCDs when the WCDs operate in the roaming system's network. Thus, when a WCD enters into coverage of a roaming system's network, the WCD may register for service with the roaming system's network and may then engage in wireless communication service via the roaming system's network. In accordance with the roaming agreement, the roaming system may then report that usage to the home system and may charge the home system for the usage. In turn, the home system may then pass that charge along to the service account of the device.
Generally, a BTS serving a particular coverage area will operate on a given carrier frequency (referred to as a “carrier”) and communicate with WCDs being served in the particular coverage area on the given carrier frequency. In a coverage area where wireless communication traffic is particularly high, a BTS serving that coverage area may operate on more than one carrier for communicating with the WCDs in that coverage area. The number of carriers employed in a given coverage area may depend on various factors, such as the volume of communication traffic expected. For example, in a congested urban location, coverage areas might be designed to employ two or more carriers, while in a more sparsely populated rural location, coverage areas might employ only one carrier.
In practice, a home system RAN may broadcast in each coverage area at least one channel list message (CLM) that specifies carriers available for use in the coverage area, so that WCDs in the coverage area can determine which carrier to use. If multiple carriers are listed, then each WCD receiving the CLM may execute a hashing algorithm to determine which carrier to use when attempting to establish a communication session (e.g., a voice call or data session) in the coverage area. The process of determining a particular carrier to use based on a hashing algorithm is referred to as “hashing,” and the particular carrier that is determined based on the hashing is referred to as the “hashed carrier.” When a WCD is under the coverage of two or more neighboring coverage areas, the WCD may receive a respective CLM from each coverage area.